What version of Kindle do you have?
If you have a Kindle 2 or DX try a hard reset - push the power switch to the right and hold it there for at least 20 - 30 seconds (time yourself - it won't reset if you don't hold it long enough). Then release and let it reset - this is VERY slow so don't panic and think nothing is happening. If you think the battery is low, be sure to plug in the power cord before you do the reset.
If you have a Kindle 1, slide the back plate off and use a paper clip to push the tiny reset button.
If none of this works, call Amazon Customer Service.